I go to the same place just about every morning. Each time, I ask for no room in my coffee. But then I pour a little out to put my milk in. They see me do this, so the next time I ask for no room in my coffee, they don't fill it up all the way. After all, they probably reason, he's just going to pour some out.
It's not really a matter of principle (yet). It's just that I prefer to have control over all of the ingredients in my morning coffee.
In other words, because I am an inexplicable cheapskate in some circumstances, and because I want my coffee-and-milk concoction to fill the whole cup, I will add milk until the cup is full, even if it ends up being more than my preferred ratio. Really, I can't help it. Sometimes, if I have to top up a few ounces, I'll use skim milk instead, and then my coffee is all watery, and then nobody's happy.
I wonder idly about whether it might be a good idea to discuss, with my team of regular baristas, the relative costs of additional labor to dispose of garbage that has a few more splashes of liquid in it versus the comparative expense of dairy product to brewed coffee, in order to somehow prove that when I ask for no room, I should get no room ... but the same effort-to-benefit analysis I'd employ in such a discussion leads me to conclude that it's probably better to say nothing and keep fighting this fight.
You can't choose some battles, but you can still figure out how to achieve a truce. Some things work out like that, I guess.